Hydrostatic extrusion

ABSTRACT

THE DISCLOSURE RELATES TO A PROCESS FOR EXTRUDING A BILLET THROUGH A DIE BY SUBJECTING THE BILLET TO THE PRESSURE OF HYDRAULIC LIQUID IN A CONTAINER AND BY APPLICATION OF A DIRECT MECHANICAL LOADING ON THE BILLET SUCH AS BY APPLICATION OF A DRAWING LOAD ON THE EXTRUDED LENGTH OF THE BILLET. A DIE HAVING A CONICAL ORIFICE IS EMPLOYED AND AS THE REAR END OF THE BILLET PASSES THROUGH THE DIE THE STRESSES REQUIRED FOR EXTRUSION OF THE END OF THE BILLET THROUGH THE DIE GRADUALLY DECREASE. THUS DURING EXTRUSION OF THE REAR END OF THE BILLET THROUGH THE DIE THE PRESSURE IN THE LIQUID CAN BE GRADUALLY REDUCED UNTIL WHEN THE REAR END OF THE BILLET PASSES OUT OF THE DIE THE LIQUID PRESSURE IS REDUCED TO A LOW PRESSURE AND A BLAST OF PRESSURISED LIQUID FROM THE DIE IS AVOIDED.

1971 P. J. THOMPSON 3, Hi1 HYDROSTATIC EXTRUSION Filed April 24, 1969 2 Sheets-Sheet 1 Nov. 9, 1971 P. J. THOMPSON 3,618,351

HYDROSTA'I'IC EXTRUSION Filed April 24, 1969 2 Sheets-Sheet m United States Patent 3,618,351 HYDROSTATIC EXTRUSION Peter John Thompson, St. Annes-on-Sea, England, as-

signor to United Kingdom Atomic Energy Authority, London, England Filed Apr. 24, 1969, Ser. No. 819,027 Claims priority, application Great Britain, May 3, 1968, 21,188/68 Int. Cl. BZlc 23/02 US. Cl. 72-60 7 Claims ABSTRACT OF THE DISCLOSURE The disclosure relates to a process for extruding a billet through a die by subjecting the billet to the pressure of hydraulic liquid in a container and by application of a direct mechanical loading on the billet such as by application of a drawing load on the extruded length of the billet. A die having a conical orifice is employed and as the rear end of the billet passes through the die the stresses required for extrusion of the end of the billet through the die gradually decrease. Thus during extrusion of the rear end of the billet through the die the pressure in the liquid can be gradually reduced until when the rear end of the billet passes out of the die the liquid pressure is reduced to a low pressure and a blast of pressurised liquid from the die is avoided.

BACKGROUND OF THE INVENTION This invention relates to extrusion processes and in particular to hydrostatic extrusion processes. In a conventional extrusion process a billet held within a container is subjected to a direct mechanical loading to ex trude the billet from the container through a die. The billet is a close fit in the container and extrusion pressure is applied on the end face of the billet by a ram operating in the bore of the container. Hydrostatic extrusion has several advantages over conventional extrusion and differs from conventional extrusion in that a liquid is used to apply extrusion pressure on the billet. The liquid envelops the billet in the extrusion container and is pressurised to act directly on the billet. Because the liquid envelops the billet there is no frictional contact between the container and the billet. Die friction is also reduced because the pressurised fluid adjacent the throat of the die provides hydrodynamic lubrication between the extruding material and the die.

An inherent difiiculty arises in operation of a hydrostatic extrusion process When a billet is fully extruded by this method, in that as the rear end of the billet passes out through the die the pressurised liquid in the extrusion container is released with considerable force which may damage the extruded product or the extrusion apparatus.

In hydrostatic extrusion the material of the billet is subjected to a hydrostatic stress system which results in extrusion of the material of the billet through the die.

The stresses set up in the billet and hence the forces acting to extrude the material of the billet through the die are dependent on the pressure app-lied in the liquid surrounding the billet. The degree of pressure which must be applied in the liquid to cause extrusion depends on the yield stress of the material to be extruded and the degree of reduction to be achieved.

In the case of material having a very high yield stress the pressure required in the liquid to effect hydrostatic extrusion may be prohibitively high because of the difliculties arising from containment of liquid subjected to such high pressure.

This latter difiiculty can be avoided by use of a mechanically augmented hydrostatic extrusion process wherein in addition to stressing of a billet by the applica tion of pressure in a liquid enveloping the billet a directly applied mechanical stress is also set up in the billet. By this means extrusion of the billet is affected using a lower pressure in the liquid enveloping the billet that is required in the liquid for operation of a purely hydrostatic extrusion process. The directly applied mechanical stress can be set up in the billet by application of a compressive mechanical loading on the billet by a ram acting on the rear end of the billet. Alternatively a tensile drawing load may be appliedon the extruded length of the billet outside the extrusion container.

The present invention provides for operation of such a mechanically augmented hydrostatic extrusion process, in a manner such that a billet can be extruded without release of a blast of high pressure liquid as the rear end of the billet passes out of the extrusion die.

SUMMARY OF THE INVENTION According to the invention in a process for extrusion of a billet through a die by directly applied mechanical stressing of the billet in combination with stressing of the billet under pressure applied in liquid enveloping the billet it is arranged that the ratio of reduction of the billet as the rear end of the billet passes through the die gradually decreases so that the stresses required to form the rear end of the billet through the die decrease correspondingly thus enabling a gradual reduction in the pressure applied in the liquid during forming of the rear end of the billet through the die.

In a particular way of carrying out the process a die is used having an orifice reducing gradually in cross section along its length such that the stresses necessary to form the rear end of the billet through the die orifice decrease gradually as the rear end face of the billet passes along the die orifice enabling a gradual reduction of the pressure applied in the liquid during forming of the rear end of the billet through the die orifice.

DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings in which FIG. 1 is a semischematic cross sectional elevation of apparatus for carrying out a hydrostatic extrusion process in accordance with the invention.

FIG. 2 is a graph.

FIGS. 2a, 2b, 2c and 2d are details of the apparatus shown in FIG. 1 at stages of operation related to the graph of FIG. 2.

'FIG. 3 is a cross section of a specialised form of die for use in carrying out the process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 of the drawings there is shown an extrusion container 1, having a longitudinal bore 2. A tubular end plug 3 is screwed in one end of the bore 2 and the other end of the bore 2 is closed by screwed plug 4, which is sealed in the bore 2 by a copper mitre ring 5 and a rubber O ring 6.

A conical entry extrusion die 7 is fitted in the bore 2 abutting the tubular end plug 3. The die 7 is sealed in the bore 2 by a copper mitre ring 8 and a rubber O ring 9. A vertically arranged cylinder 10 is seated with its bore 11 engaging with a boss 12 on the upper surface of the extrusion container 1. A cross bore 13 in the container 1 connects the bore 11 of the cylinder 10 with the bore 2 of the container 1. A liquid pressurising plunger assembly 14 is entered into the upper end of the bore 11 of the cylinder 1.

In operation of the apparatus shown in FIG. 1 liquid in the cylinder 1 is pressurised by loading of the plunger assembly 14. The pressure in the liquid 15 in the cylinder 1 is transmitted through the cross bore 13 to liquid 15 surrounding a billet 16 in the container 1. The billet 16 is extruded through the die 7 under the combined effect of the pressure P of the hydraulic liquid 15 in the container 1 and a drawing load L which is applied by a drawing device 17 on the extruded length of the billet outside the container 1.

If the billet 16 is fully extruded by the above method a blast of high pressure liquid will be released on completion of extrusion of the billet. The release of the high pressure liquid in this manner is dangerous and can cause damage to the extruded product which is ejected violently from the die.

The method of the present invention enables such a release of high pressure liquid on complete extrusion of a billet to be avoided. The operation of the method stems from the use of a die having an orifice reducing in cross section from the mouth to the throat of the orifice. The conical entry die 7 shown in FIG. 1 of the drawings is of this form.

During the extrusion of the main length of the billet 16 the stresses required for extrusion remain constant as shown by the horizontal portion A-B of the curve I in FIG. 2. Also if the pressure in the liquid 15 surrounding the billet 16 is maintained constant during extrusion of the main length of the billet 16 (as shown by the horizontal portion A-B of the curve II in FIG. 2) the augmenting stress (i.e. the drawing load) required to maintain extrusion will also remain constant. When the rear end face 18 of the billet passes through the mouth of the die 7, as shown in FIG. 20 the ratio of reduction occurring in the billet (i.e. the ratio of the area of the rear end face of the billet to the cross sectional area of the extruded length of the billet) begins to fall and continues to fall as the end face of the billet passes through the die 7. Hence as the rear end face 18 of the billet passes down the die orifice as shown in FIGS. 2a, 2b, 2c and 2d the stresses required to maintain extrusion of the end of the billet fall continuously in the manner of the portion B-E of the curve I in FIG. 2. Therefore the pressure of the liquid 15 in the container 1 can be reduced Whilst the end of the billet 16 is extruded.

At the point when the rear end face 18 of the billet passes through the mouth of the die 7 (FIG. 2c) pressure may be reduced in the liquid 15 in a continuous manner as shown by the dotted line B-E in FIG. 2. During reduction of pressure in the liquid 15 in this manner the drawing load L applied on the extruded length of the billet outside the container 1 remains constant. At the point B on the curve II in FIG. 2 the pressure in the liquid 15 has fallen to atmospheric pressure and the billet has been extruded to the point shown in FIG. 2d. The remaining length of the billet shown in FIG. 2a is then passed through the die 7 under the action of the drawing load alone, (as shown by the portion E-F in curve Iof FIG. 2).

Pressure reduction in the liquid 15 within the container 1 is achieved, for example, as is disclosed in copending British patent application No. 41,332/66 that is by gradual reduction of the loading applied on the liquid pressurising plunger assembly 14 in the arrangement of FIG. 1.

Alternatively when the rear end face 18 of the billet passes through the mouth of the die 7 the pressure in the hydraulic liquid 15 within the container 1 is initially maintained at the pressure employed in extruding the main length of the billet 16. This is as shown by the portion B- C in the curve II of FIG. 2. As the end 18 of the billet passes down the die 7 the stresses and the plastic work necessary to maintain extrusion fall. Therefore if pressure is initially maintained in the liquid 15 the drawing load necessary to maintain extrusion falls. This is as shown by the decrease in vertical distance between the portion B-C of curve I and the portion BC' of curve II in FIG. 2. The drawing load necessary to maintain extrusion is measured continuously, for example by an electrical load transducer. When the drawing load falls to a predetermined lower value, such as exists at point C on curve I of FIG. 2 pressure in the hydraulic liquid 15 within the container 1 is reduced for example from point C to point X on the curve II in FIG. 2. As the pressure in the liquid 15 falls the drawing load required to maintain extrusion rises to compensate for the reduction in pressure of the liquid 15. When the drawing load has risen to a predetermined maximum value, reduction in pressure in the liquid 15 is terminated. At this stage the billet 16 has been extruded to the point shown in FIG. 2b. The billet continues to extrude at a steady rate and the drawing load necessary to maintain extrusion again falls whilst the pressure in the liquid 15 is maintained constant at a lower value as shown by the portion XD of the curve II in FIG. 2. When the drawing load again reaches the predetermined lower value, pressure in the hydraulic liquid 15 within the container is again reduced. Pressure in the hydraulic liquid is reduced, for example from point D to point Y on curve II of FIG. 2. At this stage the billet has been extruded to the point shown in FIG. 20. This cycle of operations is then repeated so that the pressure in the hydraulic liquid 15 is finally reduced to atmospheric pressure i.e. to point E on curve II of FIG. 2 when the billet will have been extruded to the point shown in FIG. 2d. The remaining length of the billet shown in FIG. 2d is then passed through the die 7 under the action of the drawing load alone (as shown by the portion E-F in curve I of FIG. 2).

In carrying out the first of the above methods, wherein the pressure of the liquid 15 in the container 1 is reduced continuously, the pressure in the liquid 15 has to be reduced in a nonuniform manner if a die 7 of straight conical taper is employed. As can be seen from the dotted curve in FIG. 2 a greater rate of reduction of pressure in the liquid 15 is required at the commencement of extrusion of the end of the billet than is required during the later stages of extrusion of the end of the billet.

The die 7 shown in FIG. 3 of the drawings has an internal profile which enables reduction of pressure in the liquid 15 in a uniform fashion.

The die 7 shown in FIG. 3 has an internal profile in accordance with the formula,

=c0nstant tan a being the semi-angle of the die at any radius r along the length of the die.

I claim:

1. A hydrostatic extrusion process comprising the steps of enveloping a billet by liquid in an extrusion container, pressurizing the liquid to act directly on the billet and providing a direct mechanical loading on the billet to supplement the stresses acting to extrude the billet from the extrusion container through a die, said die being of the form having an orifice reducing gradually in cross section along its length such that the stresses necessary to form the rear end of the billet through the die orifice decrease gradually as the rear end face of the billet passes along the die orifice from its larger cross section inlet end to its smaller cross section outlet end, said process including the further steps of gradually reducing the pressure applied in the liquid during forming of the rear end of the billet through the die orifice and controlling the direct mechanical loading applied on the billet such that the amount of stress required to form the rear end of the billet through the die orifice is maintained as the pressure in the liquid is reduced.

2. A process for extrusion of a billet through a die as claimed in claim 1 employing a. die having an orifice reducing in cross section in accordance with the expression tan a wherein tan a is the semi-angle of the die at the cross sectional radius r of the die at any point in its length.

3. A hydrostatic extrusion process as claimed in claim 1 wherein the direct mechanical loading is applied on the billet by application of a tensile drawing load on the extruded length of the billet outside the extrusion container.

4. A hydrostatic extrusion process as claimed in claim 1 wherein, during the step of extruding the rear end of the billet through the die orifice, pressure in the hydraulic liquid is continuously reduced from the maximum operating pressure to substantially atmospheric pressure at the termination of extrusion of the rear end of the billet, the pressure in the hydraulic liquid being reduced in such a manner that the direct mechanical loading necessary to maintain extrusion as the rear end of the billet passes through the die orifice remains constant.

5. A hydrostatic extrusion process as claimed in claim 4 wherein the direct mechanical loading is applied on the billet by application of a tensile drawing load on the extrudued length of the billet outside the extrusion container.

6. A hydrostatic extrusion process as claimed in claim 1 wherein as the rear end of the billet initially passes through the larger cross section inlet end of the die orifice the pressure in the hydraulic liquid within the extrusion container is maintained at a maximum and the direct mechanical loading required to maintain extrusion of the rear end of the billet through the die. orifice is continuously measured until, when the direct mechanical loading has fallen to a pre-determined lower value, pressure in the hydraulic liquid in the extrusion container is gradually reduced so that the direct mechanical loading required for continued extrusion of the rear end of constant the billet through the extrusion die orifice increases to compensate for the fall in pressure in the liquid, reduction in pressure in the liquid being terminated when the direct mechanical loading has risen to a pre-determined maximum value, extrusion of the rear end of the billet through the die orifice continuing at a steady rate while the direct mechanical loading necessary to maintain extrusion again falls until the predetermined lower value of direct mechanical loading is reached for a second time, pressure in the hydraulic liquid then being gradually reduced again until the direct mechanical loading once again rises to the predetermined maximum value, this sequence of operations being repeated by the number of times necessary to extrude the rear end of the billet through the die orifice, the pressure in the liquid being finally reduced to substantially atmospheric pressure when the end face of the billet reaches the smaller cross section outlet end of the die orifice.

7. A hydrostatic extrusion process as claimed in claim 6 wherein the direct mechanical loading is applied on the billet by application of a tensile drawing load on the extruded length of the billet outside the extrusion container.

References Cited UNITED STATES PATENTS 3,328,998 7/1967 Sabrotf et al. 72-60 3,344,636 10/1967 Pugh 72-60 3,364,716 1/1968 Averill et al 72-60 3,455,134 7/1969 Thompson 72-60 3,491,565 1/1970 Birman 72-60 3,511,071 5/1970 Ikaheimo et al 72-60 RICHARD J. HERBST, Primary Examiner US. Cl. X.R. 72-270, 271 

